The Sony MA900 and DT990(600 Ohm) are almost on same level in term of soundquality and both offers amazing clarity with Great mids, highs and punchy bass, the only difference is MA900 are easy to drive and they sound superb out of DAPs. Sony uses 70mm dome type driver which bring out more details and it's light easy to wear for long listening sessions and very durable. On other DT990 offers great balanced soundquality but it need AMPs help to shows it best and has great imaging.

The Sony MA900 and DT990(600 Ohm) are almost on same level in term of soundquality and both offers amazing clarity with Great mids, highs and punchy bass, the only difference is MA900 are easy to drive and they sound superb out of DAPs. Sony uses 70mm dome type driver which bring out more details and it's light easy to wear for long listening sessions and very durable. On other DT990 offers great balanced soundquality but it need AMPs help to shows it best and has great imaging.

fortunately the DT 990 pro costs about 150€ at the moment. The Sony costs about 50€ more. Do you think I need an amp if I also can put the Beyer's plug into my Pioneer Reciever? On Beyerdynamic's website, it says that you should drive it with an high-fi Reciever

The DT990 can easily drive by the receiver and I connect with C421 portable AMPs and they were very loud at max vol level, so they are not that hard to drive from fairly powerful source like A/V receivers.

Yeah, that Sounds good! And sooner or later I'll buy an amp, too...
Do you think I should create a new thread "beyer dt990 vs. Sony MA900 vs. Shure SRH 840", so that everyone can see it?
(Not that noone could read it here, but with that sentence as a topic it would be more visible)